Why use Call Dword for Internal Procedures?

[Gary Beene]

Help indicates that "Call Dword" can be used for internal procedures, rather than for external DLL procedures.

Can someone discuss why you'd need to do that?

[Edwin Knoppert]

For callback situations.
I sometimes write 'standard' code where a callback can be used for custom things.

For example, UrlDownloadToFileEx() can be used with a callback, a pointer is needed.
To extend on that... if the API callback doesn't suit you, you can still call a callback which suits you better.

Anyway, a real functionname is not always present + you may have different callback behaviour.

[Michael Mattias]

Memory-Mapped File Version of LINE INPUT

PB/WIN: Write your own ENUM functions to callback September 23, 2002

[Greg Turgeon]

Another important use involves testing for something, then calling the appropriate function. Something like:

if has(HardwareX) then
   pFunction = codeptr(FunctionX)
else
   pFunction = codeptr(FunctionY)
end if

for i = 0 to maxx
   call dword pFunction using FunctionXYdeclaration(Params(i))
next i

Moving the test out of the loop is straight from Programming 101.

[Jeff Blakeney]

I've used CALL DWORD to do a jump table. Kind of like an ON x GOSUB statement.

[Fred Harris]

Call Dword

Call Dword comes in handy in all sorts of places. I guess its the newer counterpart to the old Call Absolute in older DOS basics.

I use it in all my programs (as Edwin indicated) as part of my message cracker routines like so (see fnWndProc() near bottom)...

#Compile       Exe
#Include       "Win32api.inc"
%IDC_BUTTON1   =2000
%IDC_BUTTON2   =2002

Type WndEventArgs
  wParam As Long
  lParam As Long
  hWnd   As Dword
  hInst  As Dword
End Type

Declare Function FnPtr(wea As WndEventArgs) As Long

Type MessageHandler
  wMessage As Long
  dwFnPtr As Dword
End Type
Global MsgHdlr() As MessageHandler


Sub Command1_OnClick(Wea As WndEventArgs)
  MsgBox("Thanks!  I Needed That!")
End Sub


Sub Command2_OnClick(Wea As WndEventArgs)
  MsgBox("I Don't Care To Be Clicked!")
End Sub


Function fnWndProc_OnCreate(wea As WndEventArgs) As Long
  Local pCreateStruct As CREATESTRUCT Ptr
  Local hBtn As Dword

  pCreateStruct=wea.lParam
  [email protected]
  hBtn=CreateWindow("button","Click Me!",%WS_CHILD Or %WS_VISIBLE,100,20,120,25,wea.hWnd,%IDC_BUTTON1,wea.hInst,Byval 0)
  hBtn=CreateWindow("button","But Not Me!",%WS_CHILD Or %WS_VISIBLE,100,60,120,25,wea.hWnd,%IDC_BUTTON2,wea.hInst,Byval 0)

  fnWndProc_OnCreate=0
End Function


Function fnWndProc_OnCommand(wea As WndEventArgs) As Long
  Select Case As Long Lowrd(wea.wParam)
    Case %IDC_BUTTON1
      Call Command1_OnClick(wea)
    Case %IDC_BUTTON2
      Call Command2_OnClick(wea)
  End Select

  fnWndProc_OnCommand=0
End Function


Function fnWndProc_OnClose(wea As WndEventArgs) As Long
  Call PostQuitMessage(0)
  fnWndProc_OnClose=0
End Function


Function fnWndProc(ByVal hWnd As Long,ByVal wMsg As Long,ByVal wParam As Long,ByVal lParam As Long) As Long
  Local wea As WndEventArgs
  Register iReturn As Long
  Register i As Long

  For i=0 To 2
    If wMsg=MsgHdlr(i).wMessage Then
       wea.hWnd=hWnd: wea.wParam=wParam: wea.lParam=lParam
       Call Dword MsgHdlr(i).dwFnPtr Using FnPtr(wea) To iReturn
       fnWndProc=iReturn
       Exit Function
    End If
  Next i

  fnWndProc=DefWindowProc(hWnd,wMsg,wParam,lParam)
End Function


Sub AttachMessageHandlers()
  ReDim MsgHdlr(2) As MessageHandler   'Associate Windows Message With Message Handlers
  MsgHdlr(0).wMessage=%WM_CREATE   :   MsgHdlr(0).dwFnPtr=CodePtr(fnWndProc_OnCreate)
  MsgHdlr(1).wMessage=%WM_COMMAND  :   MsgHdlr(1).dwFnPtr=CodePtr(fnWndProc_OnCommand)
  MsgHdlr(2).wMessage=%WM_CLOSE    :   MsgHdlr(2).dwFnPtr=CodePtr(fnWndProc_OnClose)
End Sub


Function WinMain(ByVal hIns As Long, ByVal hPrevIns As Long, ByVal lpCmdLn As Asciiz Ptr, ByVal iShowWnd As Long) As Long
  Local szAppName As Asciiz*16
  Local wc As WndClassEx
  Local hWnd As Dword
  Local Msg As tagMsg

  Call AttachMessageHandlers()                       : szAppName="Form2"
  wc.lpszClassName=VarPtr(szAppName)                 : wc.lpfnWndProc=CodePtr(fnWndProc)
  wc.cbSize=SizeOf(wc)                               : wc.style=%CS_HREDRAW Or %CS_VREDRAW
  wc.cbClsExtra=0                                    : wc.cbWndExtra=0
  wc.hInstance=hIns                                  : wc.hIcon=LoadIcon(%NULL, ByVal %IDI_APPLICATION)
  wc.hCursor=LoadCursor(%NULL, ByVal %IDC_ARROW)     : wc.hbrBackground=%COLOR_BTNFACE+1
  wc.lpszMenuName=%NULL
  Call RegisterClassEx(wc)
  hWnd=CreateWindowEx(0,szAppName,"Form2",%WS_OVERLAPPEDWINDOW Xor %WS_MAXIMIZEBOX,400,300,325,150,0,0,hIns,ByVal 0)
  Call ShowWindow(hWnd,iShowWnd)
  While GetMessage(Msg,%NULL,0,0)
    TranslateMessage Msg
    DispatchMessage Msg
  Wend

  Function=msg.wParam
End Function

[jcfuller]

This is how I used it a few years ago. Note this was all auto generated from a dialog resource file not hand coded.
James

'==============================================================================
TYPE DLG_10_InfoType
  MaxDlgMsgs           AS LONG
  DlgMsgs              AS DWORD PTR
  DlgProcs             AS DWORD PTR
  MaxCtlMsgs           AS LONG
  CtlMsgs              AS DWORD PTR
  CtlProcs             AS DWORD PTR
  MaxNotifyMsgs        AS LONG
  NotifyCtlIds         AS DWORD PTR
  NotifyProcs          AS DWORD PTR
  ExitFlag             AS LONG
  MaxControls          AS LONG
  CtlInfo              AS ControlInfoType PTR
END TYPE
'==============================================================================

FUNCTION _
  Create_DLG_10 ( _
    hWnd    AS DWORD _
  ) AS LONG

  DIM lpMem        AS DWORD,_
      lpDataInfo    AS DLG_10_InfoType PTR, _
      MsgIndex        AS LONG, _
      MemOffset        AS DWORD, _
      RetVal        AS LONG

  lpMem = HeapAlloc(ghHeap,%HEAP_NO_SERIALIZE OR %HEAP_ZERO_MEMORY,LEN(DLG_10_InfoType))
  IF ISFALSE lpMem THEN
    MsgBox "Not Able to Allocate Mem"
    FUNCTION = 0
    EXIT FUNCTION
  END IF
  lpDataInfo = lpMem

'---------------------------------------------------------------------------
'>> Message Crackers
  #INCLUDE "DLG_10.MSG"

  RetVal = DialogBoxParam(ghInst,BYVAL 10,hWnd,CODEPTR(Dlg_Event_Proc),lpMem)
'=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*

Message Cracker file

DLG_10.MSG file
'=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=
'DLG_10
'=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=
@lpDataInfo.MaxDlgMsgs =  9 
@lpDataInfo.MaxCtlMsgs = 5
@lpDataInfo.MaxNotifyMsgs =  0 
'---------------------------------------------------------------------------
MemOffset = HeapAlloc(ghHeap,%HEAP_NO_SERIALIZE OR %HEAP_ZERO_MEMORY,(@lpDataInfo.MaxDlgMsgs + _
                               @lpDataInfo.MaxCtlMsgs + _
                               @lpDataInfo.MaxNotifyMsgs)*8)
IF ISFALSE MemOffset THEN
  MsgBox "Message Array memory allocation error"
  FUNCTION = 0
  EXIT FUNCTION
END IF
'---------------------------------------------------------------------------
IF @lpDataInfo.MaxDlgMsgs THEN
   @lpDataInfo.DlgMsgs = MemOffset
   MemOffset = MemOffset + (@lpDataInfo.MaxDlgMsgs * 4)
   @lpDataInfo.DlgProcs = MemOffset
   MemOffset = MemOffset + (@lpDataInfo.MaxDlgMsgs * 4)
END IF
IF @lpDataInfo.MaxCtlMsgs THEN
   @lpDataInfo.CtlMsgs = MemOffset 
   MemOffset = MemOffset + (@lpDataInfo.MaxCtlMsgs * 4)
   @lpDataInfo.CtlProcs = MemOffset
   MemOffset = MemOffset + (@lpDataInfo.MaxCtlMsgs * 4)
END IF
IF @lpDataInfo.MaxNotifyMsgs THEN
   @lpDataInfo.NotifyCtlIds =  MemOffset
   MemOffset = MemOffset + (@lpDataInfo.MaxNotifyMsgs * 4)
   @lpDataInfo.NotifyProcs = MemOffset
END IF
'===========================================================================
'>> BEGIN DLG_MSGS
'===========================================================================
  MsgIndex = 0
'---------------------------------------------------------------------------
'~%WM_INITDIALOG   (%WM_INITDIALOG IS ALWAYS FIRST)
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_INITDIALOG
  @[email protected][MsgIndex] = CODEPTR(DLG_10_OnWmInitDialog)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~%WM_COMMAND
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_COMMAND
  @[email protected][MsgIndex] = CODEPTR(Ctl_Event_Proc)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~%WM_CTLCOLORSTATIC
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_CTLCOLORSTATIC
  @[email protected][MsgIndex] = CODEPTR(WM_CTLCOLOR_Proc)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~%WM_CTLCOLORBTN
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_CTLCOLORBTN
  @[email protected][MsgIndex] = CODEPTR(WM_CTLCOLOR_Proc)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~%WM_CTLCOLOREDIT
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_CTLCOLOREDIT
  @[email protected][MsgIndex] = CODEPTR(WM_CTLCOLOR_Proc)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~%WM_CTLCOLORLISTBOX
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_CTLCOLORLISTBOX
  @[email protected][MsgIndex] = CODEPTR(WM_CTLCOLOR_Proc)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~%WM_CLOSE
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_CLOSE
  @[email protected][MsgIndex] = CODEPTR(DLG_10_OnWmClose)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~%WM_DESTROY
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_DESTROY
  @[email protected][MsgIndex] = CODEPTR(DLG_10_OnWmDestroy)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~%WM_DRAWITEM
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = %WM_DRAWITEM
  @[email protected][MsgIndex] = CODEPTR(DLG_10_OnWmDrawItem)
  INCR MsgIndex
'---------------------------------------------------------------------------
'>> END DLG_MSGS
'===========================================================================
'>> BEGIN CTL_MSGS
'===========================================================================
  MsgIndex = 0
'---------------------------------------------------------------------------
'~DLG_10_CB_OK_OnClick >> %BN_CLICKED <<
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = MAKDWD(%DLG_10_CB_OK,%BN_CLICKED)
  @[email protected][MsgIndex] = CODEPTR(DLG_10_CB_OK_OnClick)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~DLG_10_CB_CANCEL_OnClick >> %BN_CLICKED <<
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = MAKDWD(%DLG_10_CB_CANCEL,%BN_CLICKED)
  @[email protected][MsgIndex] = CODEPTR(DLG_10_CB_CANCEL_OnClick)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~DLG_10_PB_BROWSE_OnClick >> %BN_CLICKED <<
'---------------------------------------------------------------------------
  @[email protected][MsgIndex] = MAKDWD(%DLG_10_PB_BROWSE,%BN_CLICKED)
  @[email protected][MsgIndex] = CODEPTR(DLG_10_PB_BROWSE_OnClick)
  INCR MsgIndex
'---------------------------------------------------------------------------
'~DLG_10_LB_1_Onlbnselchange
  @[email protected][MsgIndex] = MAKDWD(%DLG_10_LB_1,%LBN_SELCHANGE)
  @[email protected][MsgIndex] = CODEPTR(DLG_10_LB_1_Onlbnselchange)
  INCR MsgIndex
''---------------------------------------------------------------------------
'>> END CTL_MSGS
'===========================================================================
'>> BEGIN NOTIFY_MSGS
'===========================================================================
  MsgIndex = 0
'---------------------------------------------------------------------------
'>> END NOTIFY_MSGS
'===========================================================================
'~EOM         End of Messages
'=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=*=